Answer :

We have, cos 12° + cos 84° + cos 156° + cos 132°


= (cos 132°+ cos 12°) + (cos 156° + cos 84°)



= 2 cos72°.cos60° + 2 cos120°.cos36°



= cos72° - cos36°


= cos(90° -18°) – cos36°


= sin 18° - cos36°





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